The present invention relates to free piston engines.
Conventionally, internal combustion engines have operated with the motion of the pistons mechanically fixed. For example, a conventional internal combustion engine for a motor vehicle includes a crankshaft and connecting rod assemblies that mechanically determine the motion of each piston within its respective cylinder. This type of engine is desirable because the position of each piston is know for any given point in the engine cycle, which simplifies timing and operation of the engine. While these conventional types of engines have seen great improvements in efficiency in recent years, due to the nature of the engines, that efficiency is still limited. In particular, the power density is limited because the mechanically fixed motion of the pistons fixes the compression ratio. Moreover, all of the moving parts that direct the movement of the pistons (and camshafts and engine valves as well) create a great deal of friction, which takes energy from the engine itself to overcome. The resulting lower power density means that the engine will be larger and heavier than is desired. Also, the flexibility in the engine design and packaging is limited because of all of the mechanical connections that must be made.
Consequently, is desirable, for environmental and other reasons, to have an engine with a higher power density than these conventional engines. The advantages of lighter relative weight, smaller package size, and improved fuel efficiency can be a great advantage in both vehicle and stationary power production applications.
Another type of internal combustion engine is a free piston engine. This is an engine where the movement of the pistons in the cylinders is not mechanically fixed. The movement is controlled by the balance of forces acting on each piston at any given time. Since the motion is not fixed, the engines can have variable compression ratios, which allow for more flexibility in designing the engine's operating parameters. Also, since there are no conventional crankshafts and rods attached to the crankshaft, which reduces piston side force, there is generally less friction produced during engine operation. Moreover, an opposed piston, opposed cylinder (OPOC) configuration of a free piston engine is desirable due to its inherently balance operation—with a compact layout as well.
One concern, in particular, arises with an OPOC configuration of a free piston engine. The piston assemblies need to operate exactly opposed to one another. If there is unsymmetrical friction, or any other type of lasting unsymmetrical forces, these forces will cause the piston assemblies to vary from exact opposition, which, in turn, will cause the engine to cease operating after a certain period of time—the larger the asymmetry of forces, the sooner the engine will cease to operate. In a crankshaft driven engine, by contrast, the pistons can be mechanically forced to maintain the opposed motion. But in a free piston engine, only a balance of forces determines the motion of the piston assemblies. Thus, in order to obtain the efficiency benefits of an OPOC free piston engine, it is desirable to have a reliable, accurate and relatively simple way to maintain the piston assemblies in exact opposition to one another. Moreover, it is desirable, for appropriate engine operation, that the piston assemblies do not tend to drift toward one end of their travel. Otherwise, the engine operation may be adversely affected by this drift.